1. Field
Exemplary embodiments of the present invention relate to a program method of a nonvolatile memory device, and more particularly, to a program method of a nonvolatile memory device which performs a program inhibit operation using a local self-boosting scheme.
2. Description of the Related Art
A nonvolatile memory device is capable of electrically programming and erasing data and does not require a refresh function for rewriting data periodically. Here, programming refers to an operation of writing data into a memory cell.
In a NAND-type flash memory device, which is an exemplary nonvolatile memory device, adjacent memory cells share a drain or source such that a plurality of memory cells are coupled in series to form a cell string (that is, a memory cell string). NAND-type flash memory devices are suitable for storing a large amount of information.
Each of cell strings of a NAND-type flash memory device may include a drain selection transistor, memory cells, and a source selection transistor, which are coupled in series between a bit line and a common source line. The drain selection transistors, the memory cells, and the source selection transistors in different cell strings have their gates coupled to one another by a drain selection line, word lines, and a source selection line, respectively, where they are controlled by the drain selection line, the word lines, and the source selection line, respectively. Here, memory cells having control gates commonly coupled to a word line are controlled by the word line and form a page. On a larger scale, the plurality of cell strings that includes pages and a plurality of bit lines coupled to the cell strings form a memory cell block.
In programming memory cells, an erase operation may be first performed on memory cells to adjust their threshold voltages to a negative voltage. Subsequently, when a high voltage is applied as a program voltage to a word line of a selected memory cell the threshold voltage of the selected memory cell increases. On the other hand, the threshold voltages of the remaining memory cells are not changed.
However, in applying the program voltage to the selected word line during the program operation, the program voltage is applied to all memory cells sharing the selected word line. Accordingly, memory cells coupled to the selected word line other than the selected memory cell may be programmed (that is, a program disturbance).
As to methods for preventing such a program disturbance, a program inhibit method using a self-boosting scheme and a program inhibit method using a local self-boosting scheme are available.
In the self-boosting scheme, all memory cells of a cell string coupled to an unselected bit line are electrically isolated by turning off a source/drain selection transistor and enter a floating state while the memory cell channels are coupled in series. Here, a channel voltage is boosted. Since a voltage difference between the boosted channel voltage and a word line is small, a program inhibit cell (that is, a memory cell not to be programmed) coupled to the unselected bit line may be prevented from being programmed even though a program voltage is applied to a selected word line.
The local self-boosting scheme is similar to the self-boosting scheme but differs in the following features. In the local self-boosting scheme, as a memory cell positioned in one side or both sides of a selected memory cell among memory cells of a cell string coupled to an unselected bit line is turned off, memory cell channels of the cell string coupled to the unselected bit line are electrically separated into two or more local channels (that is, a channel to which the selected memory cell belongs and the other channels) that are isolated from each other and enter a floating state. Here, the channel voltages of the respective local channels are boosted. In the local self-boosting scheme, the boosting effect of the channel voltage of the program inhibit cell may be strengthened as compared with the self-boosting scheme.
However, when the local self-boosting scheme is applied, a difference between the boosted local channel voltage and a channel voltage of a turned-off memory cell may be large and thus cause hot carrier injection, where leakage current in the turned-off memory cell contributes to injection of electrons in an adjacent memory cell such as a program inhibit cell.
Here, the above-described hot carrier injection may degrade device characteristics by, for example, varying the threshold voltage of a memory cell, causing an error in a program operation or read operation, etc. Therefore, a technology for preventing the above-described hot carrier injection is useful.